1. Field of the Invention
The present invention relates in general to an optical wavelength filter, and more particularly to an optical waveguide type wavelength filter.
2. Description of the Prior Art
In the field of optical transmission, when wavelength-divisional multiplex transmission (WDM transmission) is practiced, wavelength filters are sometimes used for separating two or more different wavelengths of light transmitted over a common transmission path into different optical paths, or for introducing two or more different wavelengths of light into a common optical transmission path. The wavelength filter of the described type is generally formed by piling up on a glass substrate a plurality of layers of dielectric films with different indexes of refraction. There has been a demand for such wavelength filter of small size and with stabilized characteristics.
Conventional optical wavelength filters are structured as described above by having a plurality of layers of dielectric films, chiefly of SiO.sub.2, TiO.sub.2, etc., alternately piled up on a glass substrate. Of the dielectric-multilayer-film optical wavelength filters, those of an open structure, in which the topmost layer of the dielectric multilayer films is exposed to the air, and those of a short structure, in which another glass substrate given a non-reflective treatment is tightly attached onto the topmost layer of the dielectric multilayer films with an optical adhesive, are known. With dielectric-multilayer-film optical wavelength filters of either structure, in order that a predetermined filter characteristic is obtained, the filter is disposed in the optical path at a specified angle.
However, the problem with the above described conventional dielectric-multilayer-film optical wavelength filters is that the filter parts, in order to obtain the filter having predetermined filter characteristics, must be adjusted precisely in positioning before they are fixed and that they tend to shift their positions with the lapse of time leading to unstable filter characteristics. Further, a filter of a small size is difficult to fabricate because it is of the structure of dielectric multilayer films piled up by evaporation on a glass substrate. Besides, with the dielectric-multilayer-film optical wavelength filter, the film materials and film structure (including film thickness) must be designed specially to attain a desired filter characteristic, and therefore, there is a problem that a filter fixed for a specified wavelength can only be realized.